1952 British Penny
Although a number of in-depth and comprehensive reference books had been written on the British bronze penny, especially those by Charles Peck and Michael Freeman, it was not until October 1997 that an example of a 1952 British penny came to light in a Baldwins auction (https://www.sovr.co.uk/george-vi-1952-proof-penny-the-only-example-that-exists-pf64rb-em13448.html). The coin in question was a proof, which is unsurprising given that the 1950 and 1951 penny mintages were the lowest of the 20th century, and circulating pennies were not struck again until 1961: there was clearly no need for any circulating pennies in 1952, so any that were struck would likely have been proofs for collections.
The circumstances in which the proof were struck are unknown: a number of pattern Elizabeth II pennies were struck in 1952 (p75, Freeman, The Bronze Coinage of Great Britain (1860-1970), 2016) though these were likely for testing of the new obverse and so are likely unrelated. A single proof 1952 English shilling is also known - perhaps there were plans to strike proofs in greater numbers like in 1950 and 1951 before the death of King George VI very early in the year. In any case, the 1952 British penny appears to be unique and is one the rarest British pennies.